Field of the Invention
The present invention relates to an illumination apparatus, an image sensor unit, an image reading apparatus, and an image forming apparatus.
Description of the Related Art
In an illumination apparatus used in an image sensor unit, a rod-like light guide may shape a point light source into a line. In the light guide, a light incident surface that receives light from the light source is formed on an end surface in a longitudinal direction. A light emission surface that emits light toward an illuminated object is formed in a long and thin band shape on a side surface.
However, according to the configuration, intensity of light emitted from the light emission surface (hereinafter, “illumination intensity”) decreases with an increase in the distance from an end closer to the light incident surface. Therefore, patterns for diffusing the light are formed at predetermined intervals on the opposite side of the light emission surface of the light guide to make the illumination intensity uniform in the longitudinal direction. Examples of the patterns include prims-like structures and printed patterns for reflecting light. The intervals between the patterns are the largest immediately next to the light incident surface, and the intervals decrease with an increase in the distance from the light incident surface (see Patent Documents 1 and 2).
However, the following problem may occur with the configuration. More specifically, microscopic illumination intensity is high at a position provided with the patterns, and the intensity decreases with an increase in the distance from the patterns. Therefore, when the intervals between the patterns are large, the reduction in the illumination intensity between the patterns is large. As a result, nonuniformity of the illumination intensity is noticeable in an area with large intervals between the patterns immediately next to the light incident surface.
Particularly, if the light guide and the illuminated object approach each other for some reason, the nonuniformity of the illumination intensity is easily noticeable for the following reason. The light emitted from the light emission surface is reflected by the illuminated object, and part of the reflected light enters inside of the light guide from the light emission surface. The incident light is diffused by the patterns, and the light is emitted again toward the illuminated object from the light emission surface. Therefore, if the light guide and the illuminated object approach each other, the intensity of light incident on the light guide from the light emission surface is high, and the microscopic illumination intensity at the position provided with the patterns is also high. As a result, the nonuniformity of the illumination intensity is easily noticeable in an area with large intervals between the patterns.
Patent Document 1
Japanese Laid-open Patent Publication No. 2011-87290
Patent Document 2
Japanese Laid-open Patent Publication No. 10-112783